Microwave Journal

The Battle Between HF and UHF RFID

It is undeniable that RFID tags reign over the bar coding technologies and other optically read technologies due to its contactless nature, larger memory capacity and ability to allow energy to penetrate certain goods that are hidden or covered with su...

May 15, 2008

In the 21st century, RFID woke up from its hibernating stage and started making a great impact in the many applications of our lives. The ability of RFID to receive, modify and pass on information, and store large memories regarding any object embedded with a transponder brings a whole new dimension to various applications such as security systems, vehicles, personnel access control, asset tracking (baggage, ship container and animal tracking), production control, sports timing and document authentication. There has been much discussion about the potential of RFID as an important identification tool, especially in the retail market.


In the pharmaceutical industries, losses arising from the counterfeiting of drugs and overstocked or outdated products makes an RFID system a more sought after automatic identification system. Several surveys were done by the World Health Organisation (WHO), Industrial Surveys and USNPC:1

• 30 percent of pharmaceuticals in the developing world and 6 to 10 percent in the developed world are counterfeit (WHO)

• Up to 25 percent of a physician’s time is spent filling in forms, computer entry and data searching (Industrial Surveys)

• Non-compliance with medication in the US causes 125,000 deaths yearly and 11 percent of hospital admissions (USNPC)

• Preventable medical errors in the US cause between 44,000 and 98,000 people to die yearly (USNPC)

Recently, much of the focus has been on UHF passive tags compared to the more established HF (13.56 MHz) and LF (125 to 134.2 kHz) technologies. The 860 to 956 MHz range of UHF has attracted most of the R&D investments. All of these are probably due to the Wal-Mart RFID mandate to their suppliers to follow suit using the UHF band for case-level and pallet-level supply-chain tracking. Wal-Mart, together with Gillette and Proctor & Gamble, believe that UHF is more suitable for supply chain management at the pallet and to some degree at the case level because of the longer read range. In support of this mandate, the industry introduced in December 2004 its first global UHF RFID tag standard, known as EPC (Electronic Product Code) Gen 2 (Generation Two) ratified by EPCGlobal. Not until last year did manufacturers start developing tag and reader technology incorporating the new standard. In addition, EPCGlobal is also turning its attention to the continuing development of existing EPC standards for HF (13.56 MHz) technology. They are now focused on creating an EPCGlobal Gen 2 standard for HF bands for healthcare applications.2 So far, established global standards developed by ISO/IEC for HF in item-level tracking includes the ISO/IEC 18000-3 and ISO/IEC 15693.3 It is the author’s opinion that with all the events that are ongoing now, HF, by far, is a better option when it comes to item-level tracking and other tagging usage.

Comparison of HF and UHF

Communication Systems

HF and UHF can be significantly distinguished when wireless communication methods between the reader and the tag are examined. HF systems use the magnetic field whereas UHF systems use the electric field to transmit power and data.

In HF systems, the magnetic field powers up an RFID tag through inductive coupling of two resonant circuits tuned at frequencies as close as possible. A magnetic field is created as a result of electrical current flow in a closed loop of the reader’s antenna, often made of electrically conductive material such as copper. The reader emits the magnetic field and when a transponder passes by, the magnetic field induces an electric current flowing on the antenna of an RFID tag. The induced electric current is then used to power the RFID tag’s circuitry (passive tags) and begins to transmit its on-chip stored data. The signal generated by the reader usually provides timing information as well as enough energy to power the tag. The tag sends the data back to the reader by modulating the amplitude, frequency or phase, in accordance with the data carrying bit stream. Figure 1 shows amplitude shift keying (ASK) modulation. Data can also be modulated onto a sub-carrier. Higher frequency sub-carriers are generally used for higher data rates.

In UHF systems, the electric field powers up the tag passing within the energy field. The power of the electric field is used for the RFID tag’s circuitry in a similar way to HF tags, but through capacitive coupling instead of inductive coupling. UHF systems use backscatter modulation to communicate the data from the tag to the reader (see Figure 2). The term backscatter refers to the portion of the transmitted signal that is reflected back 180 degrees opposite the direction of the incident signal, as opposed to random scattering that is lost in space.5 In this method, the tag communicates with the reader by modulating the received signal and radiating it back to the reader. This scheme is fundamentally different than the inductive coupling method used in HF systems.

Performance Degradations from Metals and Liquids

HF has longer wavelengths, which means they are less susceptible to absorption by liquids and are able to penetrate into them. UHF’s shorter wavelengths are more susceptible to absorption by liquids. Therefore, in practical applications, HF tags are better suited for liquid-bearing products. UHF tags can be made to work, but they have to compromise with their read range, which will result in the read range drastically reduced.

Metallic environments, however, can affect all RFID frequencies. Radio frequency signals can not penetrate through metal. When metals are close to the reader’s or the tag’s antenna, the characteristics of the system are changed. Metal changes the inductance of the antenna on HF and UHF tags by basically re-tuning its resonant frequency, reducing the overall read range. The energy can also be reflected by the metal, disabling full penetration into it. However, HF tags are less susceptible to metal degradation, compared to UHF tags.

Fishnet vs. Swiss Cheese Analogy

The intensity of the electric field in UHF systems is not as well defined as the magnetic field for a specific read zone because they are affected by areas known as field nulls. This happens when item-level tags are in close physical proximity to one another or having materials with high permittivity, such as liquids, or high reflectivity, such as metals.

An analogy3 illustrates the difference between HF and UHF tags for item and pallet identification. For example, imagine the HF signal resonating as a fine fishnet wrapping around the tagged packages, and the UHF signal as a piece of Swiss cheese wrapping around the same tagged packages. The Swiss cheese, representing the field null phenomenon, requires the use of alternative techniques to compensate for these holes. The HF “fishnet” interrogating signal captures all of the tags, including those on items packed closer to the center of the package, while the UHF “Swiss cheese” misses many tags that may be positioned in the inner portion of a package because they fell into these field nulls.

The field nulls require the use of a more complex signaling scheme, involving a common technique known as frequency hopping. Due to frequency hopping’s longer read range, UHF technology may be more suitable for reading case and pallet tags from portal or conveyor antennas. HF technology’s shorter read range allows for well-defined read zones that can facilitate small shelf and item-level applications.

Read Range

Although it was mentioned earlier that HF’s magnetic read zone is well defined, the magnetic field in HF systems has a relative strength that falls off quickly as a function of distance from the antenna, resulting in a short read range of typically up to 1 m for passive tags.6 In comparison, the electric field used in UHF systems has a relative strength that extends much further, enabling a read range of up to 10 m for passive tags.6 UHF may win over HF in this context, but it has to contend with its own limitations and challenges. For example, the read range for UHF is only achievable up to 33 cm in Europe due to current power restrictions in those countries.6 Moreover, a longer read distance becomes a disadvantage in applications such as banking and access control.

Anti-collision and Simultaneous Reading of Tags

The current trend is toward passive tags having a built-in anti-collision feature that enables multiple simultaneous reading of tags with greater than 90-percent accuracy at a rate of up to 1000 tags/sec.3 In pharmaceutical applications, anti-collision allows for all 100 individually tagged bottles in a package to be identified and read instantaneously without opening the package or using a hand-held reader to scan each item. The anti-collision (simultaneous reads) feature implementation in UHF is achieved using a protocol based on bit broadcasting as opposed to HF protocol that operates based on the time slot concept. This allows for a higher number of tags to be read simultaneously in the UHF range, typically 200 tags as opposed to 50 tags with HF systems. However, the re-tuning phenomenon and the nature of RFID frequency physics, locating a tagged item within a small read area—containing multiple tagged items—can be easier with HF due to field patterns and relative tag signals.

Memory and Data Storage

Tags come in many memory sizes and capacities depending on the application and manufacturer. Several HF tags offer storage capabilities from 96 bits up to 8K bytes of memory.3 UHF products have lower tag memory sizes. Most currently manufactured UHF tags do not have user memory and only carry a 96 bit serial number.

Tag Size and Form Factor

In comparison to the passive tag sizes of the two frequencies, UHF offers a smaller tag compared to HF.3 In the pharmaceutical application perspective, it is ideal to use smaller tags for bottles, vials and tubes. However, blister-packs and multiple unit-dose packages may need larger, self-stick, flexible tags that can be laminated to paperboard, paper, plastic or other materials. Where a foil seal is required, a small stand-off that creates an air gap may be used to insulate the tag from the disruptive properties of metal. HF tags have shown satisfactory effectiveness and are well suited due to their form-factor adaptability. The form-factor adaptability has the ability to withstand liquid, pressure and temperature changes. Passive UHF tag implementations are still in their infancy, so it is unclear exactly what issues are yet to be encountered.

Global Standards and Power Requirements

Government bodies in various regions of the world regulate the bands of the radio-frequency spectrum. The HF (13.56 MHz) occupies an international, scientific and medical (ISM) band, which is available worldwide. In December 2002, Japan’s approval to harmonize the HF frequency led to the synchronization of power levels across the world.7 Unfortunately, the bandwidth of the UHF frequency (860 to 960 MHz) varies from region to region.8 The US has specified 902 to 928 MHz, while the European Union has specific 865 to 868 MHz for RFID applications. In Asia, Japan has specified 952 to 954 MHz, while in Malaysia it is 912 to 923 MHz. Only just recently, China has approved the bandwidth in the 840.25 to 844.75 MHz and 920.25 to 924.75 MHz ranges.9 This variation in frequency allocation requires that manufacturers produce country- or region-specific tags and readers, causing a potential disconnect for companies attempting to create a globally flawless international supply chain. To solve this, the standards organizations, such as EPCGlobal, started working with governments to harmonize UHF frequencies. EPCGlobal’s new Gen 2 standard allows for frequencies and power levels that comply with regional regulations while maintaining global readability. However, China has yet to follow any standards on UHF RFID and is reluctant to recognize the EPCGlobal standard compared to the well known ISO standards. In fact, the World Trade Organization (WTO) does not recognize EPCGlobal as the official RFID standards body.10 This is a big issue to UHF supply chain users because 70 percent of Wal-Mart products are manufactured in China.11

Europe’s ETSI EN 300-220 regulations impose some limitations to the usage of the UHF band. Although the power restriction of 500 mW ERP limitation has since been improved to 2 W ERP in the 869 MHz band, even with this new power level, the continued restriction on signal modulation between the tag and the reader leads to inequality in performance between US and European systems.

Another limitation is the bandwidth restriction. This bandwidth restriction results in the inability to frequency-hop the reader and also imposes a limitation on the tag anti-collision arbitration speed. The power restriction impairs the achievable reading distance while the lack of frequency hopping causes tag visibility and reading robustness to be less than optimum.7 Currently, European transmission channels are restricted to a maximum of 200 kHz in bandwidth, versus 500 kHz in North America.9 These factors show that HF could be a global solution, compared to UHF.

Cost

UHF tags cost less compared to HF due to lower memory capacity and simpler manufacturing process. However, the cost of UHF readers is expensive. In 2005, the price of these readers ranged from $2500 to $3000.12 More recently reader prices have fallen; however, it is still expensive for companies planning to implement RFID on a large scale. The main reason for the cost is the limited availability of customized components and that there is little IC integration due to the low volume of production. However, ABI Research has predicted that prices of UHF readers are expected to continue to decrease.

As for the HF tags, the cost is relatively low. It is because the antenna for a HF tag is small enough that it can be produced by printing it onto a substrate, using conductive ink and then affixing the chip. The cost for HF tags or what are also known as INLAYS is approximately $0.70 to $0.80 CDN.5 As demand increases, prices should drop significantly.

Further research by companies such as PolyIC13 is aimed at reducing the cost of the HF tag using printed circuits. They have already succeeded in producing a functioning polymer-based 8-bit RFID tag which operates at 13.56 MHz. Other companies working on printed electronics for RFID include ORFID Corp.14

Advantage of HF Over UHF

In terms of the maturity of technology, established standards and market presence, HF is far ahead of UHF. HF has been commercially available since 1995. The frequency has long been supported by two standards of the International Organization for Standardization, ISO 14443 and 15693, which define contactless smart cards. On the other hand, UHF only achieved ISO’s imprimatur in an amendment to the 180000-6 item-management standard approved in July 2006.15 Since the introduction of the global ISO/IEC 15693 standard in 1999, manufacturers have been producing hundreds of millions of HF tags and there are efforts of data protocol sharing and the necessary infrastructure to apply RFID in various sectors and applications. More than 100 companies supplying chips, inlays, labels, readers, antennas, printers and software currently support the ISO/IEC 15693 standard.7 HF technology has been commercially used in markets such as library systems, petrol kiosk systems, authentication of sporting event tickets, access control systems, textile rental and industrial laundry, in addition to research areas, such as locating tissue samples and tracking pathology samples7 with an extremely high success rate.

The RFID chip also found great importance in document authentication such as the e-passport. When queried, the embedded chip will deliver the information of the bearer. Since HF does not work over larger distances, it is safer to utilise it in e-passports as to avoid unauthorised reading from afar. When the readers are placed near the tag, they would be fairly difficult to hide thus making it almost impossible to envision a terrorist’s ability to track people’s details as they emerge from customs.

In food and pharmaceutical industries, UHF’s longer read range advantage may seem to be a better choice for this application. In fact, UHF passive tags are being used as a standard for tracing food products. However, these tags may face a problem when in the presence of water or metal. This will lead to incorrect readings of data. Plans to improve the UHF system in this context are underway, but HF (13.56 MHz) and microwave (2.45 GHz) systems may prove to be quite promising. Password protected HF tags that have extended range reaching up to ten meters are among the new advances, making them more marketable in tracking applications.10 At the Smart Labels USA Conference in 2006, Alastair McArthur of Tagsys17 supported this fact. He believes that 13.56 MHz is the most suitable operating frequency for item-level tagging because UHF, while offering good range and therefore the choice for pallets and cases, can be unreliable where metal and fluids are present as these substances will reflect the RF field, causing blindspots in the reader’s field.

ODIN Technologies,18 a consulting and testing company specializing in RFID applications, performed a head-to-head comparison of the two technologies for tagging item-level pharmaceuticals. The testing, sponsored by Unisys Corp.,1 was performed under “as deployed” conditions, such as would be found in an actual tagging/verification process at a pharmaceuticals manufacturer. Of the eight criteria on which the testing was based, HF won five outright; UHF won two, and one was a tie. HF proved to be the ideal frequency for item-level tagging, especially where water and form factor of the tag is a concern in pharmaceutical applications. Figure 3 shows the comparison between the two frequencies.

Environmental condition is another factor that supports HF usage over UHF. HF’s inductive coupling reduces potential wireless interference issues because no real power is being radiated. In other words, HF has an excellent immunity to environmental noise and electromagnetic interference (EMI). UHF’s far-field technology radiates real power, and its higher signal strength makes it more prone to EMI. Thus, the US Food and Drug Administration (FDA) shows great concern over the EMI caused by RFID usage in hospitals. The solution would be to use the HF RFID. They have met the electromagnetic radiation limits of 3 V/m, as shown in Table 1, indicating that EMI would not interfere with critical medical devices. For years, HF tags have been used in hospitals.

In addition to that, Italy, Turkey and France’s 865.6 to 867.6 MHz frequency range conflicts with the band allocated to the tactical relays for military applications.8 This means that the UHF RFID systems may cause interference on military applications and telecommunication devices. Research20 shows that there is such a risk. Tables 2 and 3 show the very significant interference between a transmitting RFID system and a receiving military telecommunication device placed in line of sight and another out of line of sight. The European improved power level of 2 W proved to be more harmful. However, there is no risk that a RFID system could cause destructive interference to a generic military device, as shown in Table 4. Table 5 shows the summary of comparison between the HF and UHF RFID systems as previously explained in detail.

Conclusion

There is not a single, universal RFID frequency that is capable of working in all applications. Different RFID technologies will be complementing each other, each used in applications that most suit its characteristics. For example, UHF is best used in logistics, baggage tagging and case-pallet tracking where longer read range is needed, whereas HF is best used in item-level tagging and in areas where liquid and metals are involved.

Some UHF users had been discussing using a hybrid solution known as the NearField UHF tag21 to address the concern over the vicinity reading of the item-level tagging in the near future. However, two-in-one NearField/FarField UHF tags are usually too big for many items. Futhermore, UHF tags for pallets, cases and air baggage will always need to come in many variations to be effective.

For the time being, HF is still the best suited technology in most commercially growing applications because of the reasons stated in this article. The current market shows that 13.56 MHz has been the choice for many item-level applications where its range is adequate. Although Wal-Mart mandates UHF for item-level drugs, there are many leading drug companies, libraries and laundries that fit HF RFID on their items.

The most significant disadvantage of UHF is where tags are exposed to environmentally challenged conditions, where liquid or metal are present that very commonly occurs in shipping of goods, and even airline baggage tagging in which UHF dominates. HF may fail in the read range category but it is a somewhat better choice between the LF and UHF, providing the best compromise between the advantages and disadvantages.

References

1. Smart Healthcare USA Conference, 10–11 June 2004, San Francisco, CA.

2. “EPCGlobal Announces Formation of New Standards Work Group to Study High Frequency Technologies to Support Broad Industry Adoption of RFID,” Business Wire, May 4, 2006.

3. “Comparing HF and UHF RFID Technologies,” White Paper; excerpted by L.R. Hartman, authored by Philips Semiconductors, Tagsys and Texas Instruments Inc., http://www.packagingdigest.com/articles/200411/38.php.

4. “Compatibility Between Inductive LF and HF RFID Transponder and Other Radio Communication Systems in the Frequency Ranges,” Electronic Communications Committee (ECC), 2002.

5. “Understanding Radio Frequency Identification (Passive RFID),” November 2004, http://www.rmoroz.com/rfid.html.

6. “Item-level Visibility in the Pharmaceutical Supply Chain: A Comparison of HF and UHF RFID Technologies,” White Paper; Philips Semiconductors, TAGSYS and Texas Instruments Inc., July 2004.

7. “RFID: HF versus UHF Technologies, Part Two,” Philips Semiconductors, TAGSYS and Texas Instruments Inc., http://www.packagingdigest.com/articles/ 200503/36.php.

8. “Regulatory Status for Using RFID in the UHF Spectrum,” EPCGlobal, February 2, 2006, http://www.epcglobal.tobb.org.tr/rfid_UHF_Regulations20060202.pdf.

9. C. Swedberg, “China Approves Requirements for UHF Bandwidth,” RFID online journal news, http://www.rfidjournal.com/article/articleview/3318.

10. B. McCrea, “RFID: The Next Generation,” May 1, 2005, http://www.logisticsmgmt.com/article/CA600491.html.

11. “The RFID Business Planning Service,” White Paper; Venture Development Corp., http://transpondernews.com/vdc2005.pdf.

12. ABI Research, December 12, 2005, http://www.rfidlowdown.com/2005/12/abi_research_sa.html.

13. “RFID: Reducing the Costs with Printed Circuits,” September 26, 2006, http://www.rfidlowdown.com/2006/09/rfid_reducing_t.html.

14. “Organic Electronics in RFID Tags,” July 8, 2006, http://www.rfidlowdown.com/2006/07/organic_electro.html.

15. D. Essex, “RFID Technologies Duke it Out,” Government Computer News 25.27, Vol. 29, No. 5, September 11, 2006.

16. “Developments in RFID Tags Help Food and Pharmaceutical Industries,” January 25, 2006, http://www.rfidlowdown.com/2006/01/developments_in.html.

17. “Don’t Forget Item-level Tracking,” RFID Analyst, Issue 39, April 2004, http://rfid.idtechex.com/documents/en/printview.asp?documentid=104.

18. “Pharmaceutical RFID: Battle of the Frequencies,” ODIN Technologies, April 3, 2006, http://www.odintechnologies.com/165/newsarticle.html.

19. “Unisys and ODIN Technologies’ Research Reveals RFID High- vs. Ultra-high Frequency Leader for Pharmaceutical Industry,” April 4, 2006, http://www.unisys.com/commercial/news_a_events/all__news/04048642.htm.

20. A. Moro, “Potential Interference Generated by UHF RFID Systems on Military Telecommunication Devices,” http://www.rfidconvocation.eu.

21. “Research and Markets: Wal-Mart Mandates UHF for Item-level Drugs,” Business Wire, May 25, 2006.

Faisal Mohd-Yasin received his BSc degree in electrical engineering, and his MSc degrees in telecommunications and computers, and computer engineering from The George Washington University, Washington, DC. He is currently a senior lecturer on the faculty of engineering at Multimedia University, Malaysia. He also serves as an associate research fellow at the Institute of Microengineering and Nanoelectronics (IMEN) of the National University of Malaysia and as an associate at the Malaysian Industry-Government Group for High Technology (MiGHT), Putrajaya. His current research interests include circuit design and noise in micro-electromechanical systems (MEMS).

Mei Kum Khaw received her BSc degree in computational physics and electronics from the University of Malaysia, Kuala Lumpur, in 2003. She is currently pursuing her M.EngSc degree at Multimedia University, Malaysia. Her research interests include analog/digital VLSI design, RFID systems and ESD protection design.

Florence Choong Chiao received her B.Eng degree in electronics engineering and her MEngSc. degree in VLSI from Multimedia University, Cyberjaya, Malaysia, in 2002 and 2005, respectively. She is presently on the faculty of engineering at Multimedia University, Malaysia. Her current research interests include AI and VLSI design. She is currently pursuing her PhD degree.

Mamun Bin Ibne Reaz received his D.Eng. degree from Ibaraki University, Japan, and his BSc and MSc degrees in applied physics and electronics from the University of Rajhashi, Bangladesh. He is currently a lecturer in the department of electrical and computer engineering at the International Islamic University Malaysia, Malaysia. He has published extensively in the area of IC design and biomedical application IC. He is author or co-author of more than 100 papers in design automation and IC design for biomedical applications.